Attached Link: Xenofex info from Alien Skin

And you can just paint them buy hand, ofcourse. With example photo's of real world lightning an shift-clicking with brushes in decreasing sizes. Copying that layer and blurring it et all. For the purists who do not want to use stuff they did not make themself. btw, Xenofex is nice of you are far-from-purist, but it will cost you some money. And it is hardly ever used professionally. See link